The research objective was to assess the social-emotional functioning of young children of affectively ill and well parents, taking into account the developmental periods at which specific problems appear and the course of their development. The families were classified by mother's diagnoses: bipolar illness (N=22), unipolar depression (N=41) and normal control (N=37). Father's diagnosis was also obtained. Pairs of siblings were studied. The younger child was between 1 1/2 and 3 1/2 years and the older sibling was between 5 and 8 years when the study began. Two sources of information were utilized: psychiatric evaluations and mother's reports. Children were evaluated on dimensions of disruptive behavior, anxiety, and depressive characteristics. By middle and late childhood, significantly more children of affectively ill mothers exhibited depressive and disruptive behavior than did children of well mothers. Offspring of affectively ill mothers were also more likely to have multiple problems. Offspring of unipolar mothers compared with offspring of bipolar mothers developed problems at earlier ages, and problems were more likely to appear in both siblings. Time or development was a significant influence on the frequencies with which disruptive, depressive, and anxious problem-level behavior appeared, and in ways that differed by mother's diagnosis.